This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-319859, filed Nov. 10, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a photosensor system having a photosensor array constituted by two-dimensionally arraying a plurality of photosensors, and a drive control method thereof.
As a conventional two-dimensional image reading apparatus for reading print, a photograph, or a fine three-dimensional pattern like a fingerprint, some structures have a photosensor array constituted by two-dimensionally arraying photosensors (light receiving elements) in a matrix. This photosensor array generally employs a solid-state imaging device such as a CCD (Charge Coupled Device).
As well known, the CCD has a structure in which photosensors such as photodiodes, or thin film transistors (TFT: Thin Film Transistor) are arranged in a matrix, and the charge amount of pairs of electrons and positive holes generated corresponding to the amount of light entering the light receiving section of each sensor is detected by a horizontal scanning circuit and vertical scanning circuit to detect the luminance of radiation.
In a photosensor system using such a CCD, it is necessary to provide each scanned photosensor with a selective transistor for causing the scanned photosensor to assume a selected state. This increases the system size as the number of pixels increases. To prevent this, a photosensor (to be referred to as a double-gate photosensor hereinafter) is now being developed, which is formed of a thin film transistor having a so-called double-gate structure and has both a photosensing function and selecting function.
FIG. 18A is a sectional view showing the structure of a double-gate photosensor 10. FIG. 18B is a circuit diagram showing the equivalent circuit of the double-gate photosensor 10. As shown in FIG. 18A, the double-gate photosensor 10 comprises a semiconductor layer 11 formed of amorphous silicon or the like in which pairs of electrons and positive holes are generated upon reception of visible light, n+-silicon layers 17 and 18 respectively formed at the both ends of the semiconductor layer 11, source and drain electrodes 12 and 13 respectively formed on the n+-silicon layers 17 and 18, a top gate electrode 21 formed above the semiconductor layer 11 via a block insulating film 14 and upper gate insulating film 15, and a bottom gate electrode 22 formed below the semiconductor thin film 11 via a lower gate insulating film 16. The double-gate photosensor 10 is provided on a transparent insulating substrate 19 formed of glass or the like. In FIG. 18A, the top gate electrode 21, the upper gate insulating film 15, the lower gate insulating film 16, and a protective insulating film 20 formed on the top gate electrode 21 are made of materials having high transmittances for visible light which excites the semiconductor layer 11. To the contrary, the bottom gate electrode 22 is made of a material which shields transmission of visible light, and has a structure of detecting only irradiation light incident from above the structure in FIG. 18A.
The double-gate photosensor 10 can be considered to be a structure which is formed, on the transparent insulating substrate 19 of glass or the like, from a combination of two MOS transistors using the semiconductor layer 11 as a common channel, i.e., an upper MOS transistor made up of the semiconductor layer 11, source electrode 12, drain electrode 13, and top gate electrode 21, and a lower MOS transistor made up of the semiconductor layer 11, source electrode 12, drain electrode 13, and bottom gate electrode 22. This double-gate photosensor 10 can generally be represented by an equivalent circuit as shown in FIG. 18B. In FIG. 18B, TG represents a top gate terminal; BG, a bottom gate terminal; S, a source terminal; and D, a drain terminal.
FIG. 19 is a schematic view showing a photosensor system constituted by two-dimensionally arraying double-gate photosensors. As shown in FIG. 19, the photosensor system is roughly constituted by a photosensor array 100 that is comprised of a large number of double-gate photosensors 10 arranged in an nxc3x97m matrix, top and bottom gate lines 101 and 102 that connect the top and bottom gate terminals TG and BG of the double-gate photosensors 10 in a row direction, top and bottom gate drivers 111 and 112 respectively connected to the top and bottom gate lines 101 and 102, data lines 103 that connect the drain terminals D of the double-gate photosensors in a column direction, and an output circuit section 113 connected to the data lines 103. xcfx86tg and xcfx86bg represent control signals for generating a reset pulse xcfx86Ti and readout pulse xcfx86Bi, respectively, which will be described later, and xcfx86pg represents a pre-charge pulse for controlling the timing at which a pre-charge voltage Vpg is applied.
In the above-described structure, the photosensing function is realized by applying a voltage from the top gate driver 111 to the top gate terminals TG, while the selecting/readout function is realized by applying a voltage from the bottom gate driver 112 to the bottom gate terminals BG, then sending a detection signal to the output circuit section of the output circuit section 113 via the data lines 103, and outputting serial data Vout.
FIGS. 20A to 20D are timing charts showing a method of controlling the photosensor system, and showing a detecting period (i-th row processing cycle) in the i-th row of the sensor array 100.
First, a high-level pulse voltage (reset pulse; e.g., Vtgh=+15V) xcfx86Ti shown in FIG. 20A is applied to the top gate line 101 of the i-th row, and during a reset period Treset, reset operation for discharging the double-gate photosensors 10 of the i-th row is executed.
Subsequently, a bias voltage xcfx86Ti of low level (e.g., Vtgl=xe2x88x9215V) is applied to the top gate line 101, thereby finishing the reset period Treset and starting a charge accumulating period Ta in which the channel region is charged. During the charge accumulating period Ta, charges (positive holes) corresponding to the amount of light entering each sensor from the top gate electrode side are accumulated in the channel region.
Then, a pre-charge pulse xcfx86pg with a pre-charge voltage Vpg shown in FIG. 20C is applied to the data lines 103 during part of the charge accumulating period Ta, and after a pre-charge period Tprch for making the drain electrodes 13 keep charges, a bias voltage (readout pulse xcfx86Bi) of high level (e.g., Vbgh=+10V) shown in FIG. 20B is applied to the bottom gate line 102. Then, the double-gate photosensors 10 of the i-th row are turned on to start a readout period Tread.
During the readout period Tread, the charges accumulated in the channel region serve to moderate a low-level voltage (e.g., Vtgl=xe2x88x9215V) which has an opposite polarity of charges accumulated in the channel region and is applied to each top gate terminal TG. Therefore, an n-type channel is formed by the voltage Vbgh at each bottom gate terminal BG, the voltage VD at the data lines 103 gradually reduces in accordance with the drain current with lapse of time after the pre-charge voltage Vpg is applied. More specifically, the change trend of the voltage VD at the data lines 103 depends upon the charge accumulating period Ta and the amount of received light. As shown in FIG. 20D, the voltage VD tends to gradually reduce when the incident light is dark, i.e., a small amount of light is received, and hence only small charges are accumulated, whereas they tend to suddenly reduce when the incident light is bright, i.e., large amount of light is received, and hence large charges are accumulated. From this, it is understood that the amount of radiation can be calculated by detecting the voltage VD at the data lines 103 a predetermined period after the start of the readout period Tread, or by detecting a period required until the voltage VD reaches a predetermined threshold voltage.
Image reading is performed by sequentially executing the above-described drive control for each line of the sensor array 100, by executing drive control for each line in a parallel manner at different timings at which the driving pulses do not overlap.
Although the case of using the double-gate photosensor as a photosensor has been described above, even a photosensor system using a photodiode or phototransistor as a photosensor has operation steps: reset operationxe2x86x92charge accumulating operationxe2x86x92pre-charge operationxe2x86x92reading operation, and uses a similar drive sequence.
The conventional photosensor system as above has the following problems.
To read a subject image in various use environments in a photosensor system having a two-dimensional photosensor array as described above, the reading sensitivity must be properly set. The proper reading sensitivity changes in accordance with changes in ambient conditions such as the illuminance of external light in a use environment, and also changes when the characteristics of the photosensor change. In the prior art, a circuit for detecting the illuminance of external light must be additionally arranged. Alternatively, reading operation (pre-reading operation) of changing the reading sensitivity to a plurality of stages using a subject such as a normal sample before the start of normal reading operation of a subject image must be executed to obtain the optimal value of the reading sensitivity from the read result. However, a reading sensitivity setting method of uniquely and automatically setting a proper charge accumulating period based on a read result every charge accumulating period that is obtained by pre-reading operation has not been developed yet.
In addition, when the detection area of the photosensor array is larger than a subject such as a standard sample used in pre-reading operation, or a position where a subject is placed offsets from a normal position to fail to cover part of the detection area with the subject, an image read by pre-reading operation contains a background image together with a subject image. In this case, the image of the read background may influence image processing in obtaining the optimal value of the reading sensitivity, failing to set a proper reading sensitivity. If this photosensor system is applied to, e.g., a fingerprint reading apparatus, problems such as abnormal fingerprint authentication arise.
It is an object of the present invention to provide a reading sensitivity setting method of uniquely and automatically setting a proper reading sensitivity on the basis of read results obtained immediately before the start of normal reading operation of a subject image in order to accurately read a subject image in various use environments in a photosensor system having a photosensor array constituted by two-dimensionally arraying a plurality of photosensors. It is another object of the present invention to prevent any malfunction in setting the reading sensitivity even when a position where a subject is placed in the detection area of the photosensor array offsets from a normal position in performing reading operation of a subject image for setting the sensitivity.
To achieve the above objects, a photosensor system according to the present invention comprises a photosensor array constituted by two-dimensionally arraying photosensors, a driver circuit for supplying a drive signal to the photosensors, a controller for controlling reading operation of a subject image and sensitivity setting, and a RAM for storing read image data, data relating to sensitivity setting processing, and the like.
A reading sensitivity setting method according to the present invention comprises reading image data of a subject by performing pre-reading operation while changing the image reading sensitivity at a plurality of stages for, e.g., respective rows immediately before the start of normal reading operation of a subject image, calculating the absolute difference value between adjacent pixels of lightness data for each image reading sensitivity, extracting a maximum absolute difference value for each image reading sensitivity from calculated absolute difference values, extracting an image reading sensitivity having a maximum representative difference value from extracted representative difference values for image reading sensitivities, and setting the extracted image reading sensitivity as an optimal reading sensitivity.
Even when ambient light changes or the characteristics of the photosensor change, an optimal image reading sensitivity can be set in accordance with the changes. Moreover, even when a position where a subject is placed in the detection area of the photosensor array offsets from a normal position in performing pre-reading operation, and the read image data contains a background pattern together with the subject image, the absolute difference value of lightness data between adjacent pixels can be used to discriminate the subject which is placed in tight contact with the photosensor array to allow clearly reading a bright/dark pattern from the background pattern in which the image is out of focus not to clearly read a bright/dark pattern. Thus, a proper image reading sensitivity can be extracted and set without any influence of the background pattern. An image reading sensitivity setting method having high reliability can be provided.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.